TIMBEE 
13 
the long diameter is in the direction of the longitudinal 
axis of the tree, it will be understood why the shrinkage of 
boards and timber generally is inappreciable in a long 
length. Some of these cells have thick walls, others have 
thin walls, and as both kinds are generally mixed the 
shrinkage is greater in one case than in the other, causing 
different strains to occur. Again, the cells forming the 
medullary rays, which 
constitute a consider- 
able proportion of all 
wood, have their length 
at right angles to the 
others (see Fig. 5) and 
like the others shrink 
most in the direction 
of their breadth, so 
that two severe strains 
at right angles to each 
other are set up, and, 
with those previously 
referred to, cause the 
splitting and cracking 
which occur when 
timber is being sea- 
soned. If done too 
rapidly the cells have not time to " give " or adapt them- 
selves to the altered circumstances ; they became distorted 
or ruptured, and the wood is damaged. 
Shrinkage of wood then is caused by the walls of the 
cells or pores becoming thinner, and as the thicker walled 
cells are found in the harder woods, these as a rule shrink 
and split the most. Wood shrinks or twists to an extent 
dependent upon the quality and size of the timber ; this is 
more evident if it be dried too rapidly, is more noticeable in 
mr' 
Fig. 5. — a, 6, longitudinal cells or 
wood fibres ; c, d, cells of medul- 
lary ray. 
